The mammalian respiratory system is not a single structure but rather a series of linked structures, each contributing to the overall resistance (or finite capacity) for oxygen transport. Some of the structural resistors in the cascade have considerable phenotypic plasticity and hence are acutely adaptable to the level of 02 flow. Others of the structural resistors are nearly lacking phenotypic plasticity; these must be designed (and maintained) at a level that will satisfy peak potential species-specific demands. The first aim of this study is to quantify the relationship between 02 demand and 02 supply through the respiratory system. Which of the structural resistors is (are) overbuilt and which acts to limit 02 flow? The second aim of this study is to quantify two of the upstream resistors and measure their structural and functional responses to induced increases in 02 demand via long-term endurance training. We propose to quantify pulmonary diffusing capacity and cardiovascular 02 delivery in response to long-term endurance training. The final aim of this study is to couple an examination of muscle biochemistry and ultrastructure to "whole animal" 02 demand and quantify these properties during and following endurance training. These goals will be accomplished experimentally by coupling a long-term endurance training study (in domestic goats) with final experiments in which the fractional content of inspired 02 and hemoglobin concentration will be simultaneously manipulated. Untrained sibling pairs will act as controls during the study. The resultant physiological data will be coupled with measurements of pulmonary diffusing capacity, lung function tests, muscle ultrastructure, and muscle biochemistry. While this study is not proposed as the definitive study, it should provide some missing information because it utilizes at least two uncommon approaches. First, we will examine multiple (though not all) transport steps simultaneously. Second, we will couple biochemical, anatomical and physiological data from the same animals. Finally, we propose to examine these structures and functions during the course of long- term endurance training. These are the strengths of the study: a coupling of physiological and biochemical data and an experimental (vs. descriptive only) approach to examine the design consequences of endurance training.